The present invention relates generally to lasers based on chiral structures, and more particularly to optically pumped lasers utilizing chiral fiber elements.
Semiconductor lasers have found many industrial and commercial applications in recent years. For example lasers are used in telecommunications, in optically readable media pickups that are used in CD players, CD ROM drives and DVD players, in medical imaging, and in video displays. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, so it is necessary to cleave a wafer into chips and package the chip before knowing if the laser functions properly. Other types of light sources, such as LEDs do not provide the performance needed for certain applications.
Vertical Cavity Surface Emitted Lasers (hereinafter xe2x80x9cVCSELsxe2x80x9d) have been developed to address the need for a more advanced, higher quality laser that can function well in a variety of applications. VCSELs combine the performance advantages of LEDs and edge-emitting lasers at costs comparable to LED solutions. VCSELs emit light vertically from the wafer surface, like LEDs, which means their fabrication and testing is fully compatible with standard I.C. procedures and equipment, and also means that arrays of VCSELs are feasible. Additionally, VCSELs are much faster, more efficient, and produce a smaller divergence beam than LEDs.
The VCSEL structure leads to a host of performance advantages over conventional semiconductor lasers.
1) small size
2) low power consumption
3) two-dimensional array capabilities
In contrast to conventional edge-emitting semiconductor lasers, the surface-emitting VCSEL has a radially symmetric Gaussian near-field, greatly simplifying coupling to optical elements or fibers. In addition, VCSEL technology allows the fabrication of two-dimensional laser arrays.
However, VCSELS suffer from a number of disadvantages. The manufacture of VCSELs requires sophisticated and expensive mircofabrication. Since single-pass gain in thin layer semiconductor lasers is low, VCSELs incorporate highly reflective dielectric stacks which are integrated into the laser as Bragg reflectors. These consist of alternating layers of dielectric material, which are grown using methods of molecular beam epitaxy (MBE). This ensures a close match of the atomic lattice structures of adjacent layers. Alternating atomically ordered layers of materials with different electronic characteristics are thereby produced. The interfaces between the layers must be digitally graded and doped to reduce the electrical resistance.
Much work has been done to improve the performance of VCSELs by increasing the number of layers and/or the dielectric difference between alternating layers. However, this approach makes the fabrication more expensive and difficult. There is also a limit to the number of layers determined by absorption in these layers. While VCSELs can be manufactured in two-dimensional arrays, there has been great difficulty in achieving uniform structure over large areas and in producing large area arrays. The materials typically used for VCSELs do not have the desired low absorption and high index contrast over a the frequency range of interest in telecommunications. In particular, it is difficult to achieve high reflectivity in the communication band around 1.5 microns.
In addition, VCSELs cannot be tuned in frequency since their periods cannot be changed. The density of photon modes is not changed appreciably by use of a low index contrast multilayer Bragg reflector and the gain cannot be improved in a VCSEL system as compared to that in an ordinary laser cavity. Also, an external device must be used to control the polarization of the light.
A number of novel solutions have recently been developed to address the drawbacks of VCSEL-based lasers. These new techniques advantageously utilize certain chiral materials, such as cholesteric liquid crystals, to achieve high efficiency, low threshold lasers. All of these solutions are based on utilizing the photonic band gap phenomenon that occurs in periodic structures.
For example, the commonly assigned co-pending U.S. patent application entitled xe2x80x9cStop Band Laser Apparatus and Methodxe2x80x9d, discloses a novel band gap laser with increased output power and low lasing threshold with improved control over the spatial, spectral, and temporal lasing parameters. A commonly assigned U.S. Pat. No. 6,404,789 entitled xe2x80x9cChiral Laser Apparatus and Methodxe2x80x9d discloses a variety of electrically and optically pumped chiral lasers based on cholesteric liquid crystal (CLC) structures. The CLC lasers disclosed in this patent application advantageously overcame the drawbacks of previously known edge-emitting lasers and VCSELs due to unique properties of chiral (cholesteric) materials. Specifically, the disadvantages of the prior art were overcome as follows:
1) In contrast to multi-layered structures, such as VCSELs, that are difficult to produce, the CLC films/layers are self-organized structures that are readily fabricated;
2) The period of a CLC film/layer could be readily changed by applying an electric or magnetic field or changing temperature or pressure so that the laser output could be tuned in frequency within the gain band of the light-emitting middle layer;
3) The band structure of a CLC film/layer leads to an increase in the local density of photon modes over a narrow wavelength range. This in turn results in an improvement in gain and to a reduction of the lasing threshold; and
4) The polarization of the laser output is determined by the CLC structure. Thus the laser beam is right or left circularly or linearly polarized without requiring any external device.
Another commonly assigned U.S. Pat. No. 6,411,635 entitled xe2x80x9cMethod of Mode Selection in a Photonic Band Edge Laserxe2x80x9d, further improves the previously described commonly assigned patents by disclosing an apparatus and method for advantageously enabling single-mode lasing at higher pump power, for reducing the bandwidth of the lasing radiation, and for enabling advantageous selection of a particular photonic mode for lasing at that mode in a periodic laser.
The previously described patents and co-pending application also disclosed that a defect may be introduced into a periodic structure causing lasing to advantageously occur at a wavelength corresponding to a localized defect photonic state within the photonic stop band. This leads to enhanced energy density within the periodic structure. However, it is difficult to construct a layered structure with a precise light emitting material thickness required to produce a defect (the required thickness must be equal to the wavelength of light in the medium divided by 4). More importantly, the position of the localized state caused by the defect cannot be easily controlled because the thickness of the light-emitting material cannot be changed once the device is manufactured. To address this challenge, a commonly assigned U.S. Pat. No. 6,396,859 entitled xe2x80x9cChiral Twist Laser and Filter Apparatus and Methodxe2x80x9d, which is hereby incorporated by reference in its entirety, introduced a novel apparatus and method for inducing a variable (i.e. tunable) defect into a chiral structure.
The essence of the inventive techniques disclosed in the above-described commonly assigned co-pending patent applications relies on utilization of properties of one-dimensional periodic structures. There are two previously known types of one-dimensional (1D) photonic band gap (PBG) structures: (1) periodic layered media and (2) cholesteric liquid crystals (CLCs). In both of these systems the wavelength inside the medium at the center of the band gap is twice the period of the structure in question. In CLC structures, the band gap exists only for the circular polarized component of light, which has the same sense of rotation as the structure. The second circular component is unaffected by the structure. The first type of structure has been implemented in optical fibers and is known as a fiber Bragg grating (FBG). However, the second type of structures xe2x80x94CLCsxe2x80x94does not exist in form of fibers. Fiber Bragg gratings have many applicationsxe2x80x94fiber components form the backbone of modern information and communications technologies and are suitable for a wide range of applicationsxe2x80x94for example in information processing. However, FBGs based on conventional periodic structures are not easy to manufacture and suffer from a number of disadvantages.
Because CLCs exhibit superior properties in comparison to layered media (as disclosed in above-incorporated U.S. patent entitled xe2x80x9cChiral Laser Apparatus and Methodxe2x80x9d), new techniques were developed to implement the advantageous optical properties of a cholesteric periodic photonic band gap (hereinafter xe2x80x9cPBGxe2x80x9d) structure in an optical fiber which is easier to fabricate. These novel techniques for implementing CLC-like PBG structures in optical fibers and/or for fabricating such fibers are disclosed in commonly assigned co-pending U.S. Patent applications entitled xe2x80x9cApparatus and Method for Manufacturing Fiber Gratingsxe2x80x9d, xe2x80x9cApparatus and Method of Manufacturing Helical Fiber Bragg Gratingsxe2x80x9d, xe2x80x9cApparatus and Method for Fabricating Helical Fiber Bragg Gratingsxe2x80x9d, and xe2x80x9cHelical Fiber Bragg Gratingxe2x80x9d which are all hereby incorporated by reference in their entirety. These approaches captured the superior optical properties of cholesteric liquid crystals while facilitating the manufacture of the structure as a continuous (and thus easier to implement) process.
As previously described, lasers based on CLC-like chiral structures offer significant advantages over previously known periodic lasers. However, there are a number of challenges in fabricating CLC structures suitable for use in lasers. Conversely, FBG structures offer advantages in relatively simple and easy to reproduce FBG fabrication process. Utilizing the above-described techniques for fabricating FBGs with CLC-like optical properties it would be desirable to provide a chiral fiber laser that combines the advantages of chiral (e.g. CLC) and FBG structures. It would further be desirable to provide a chiral fiber laser that may be configured and fabricated for use at a specific desired wavelength. It would also be desirable to provide a chiral fiber laser having a dynamically tunable lasing wavelength.